Save/Publish Workflow does not react

yesterday I built a workflow and published the workflow. Today I wanted to change a workflow action but when I tried to save or publish the workflow the system does not react. It seems like the workflow got saved/published but after some seconds the 'save window' (picture below) appears again. The changes do not get saved. This reaction always appears by pushing the 'send'-button.

I noticed that the checkbox 'overwrite available version' below the title field is missing. Example from another workflow:

This reaction only appears by saving/publishing this workflow. Saving/Publishing other Workflows do not have any problems.

maybe something got messed up internally while creating the workflow?! I would start debugging by exporting the workflow, deleting all references on this site (the list workflow in the lists workflow page and all associated information in the "NintexWorkflows" library) and recreating it by importing your exported nwf-file.

Also try to have a look in the ULS log for the time you try to save/publish the workflow. Maybe you can find some useful information there. Keep us informed!